1. Field of the Invention
This invention relates to a ring oscillator used for a charge pump circuit or the like, and relates to a constant current circuit suitable for use as a current generator of an inverter in the ring oscillator.
2. Description of the Related Art
A charge pump for providing voltage boost by means of control of charging a capacitor has conventionally been known. Such a charge pump can be used for generating a high voltage necessary for clearing a memory such as a nonvolatile memory.
In a charge pump using CMOS as shown in FIG. 1, for example, a source voltage VDD is supplied to a source of an N channel transistor (MOS) 10. A drain of the N channel transistor 10 is connected to one terminal of a shift capacitor 12 which is supplied with a pulse voltage on the other terminal. The drain of the N channel transistor 10 is also connected to a drain of a P channel transistor (MOS) 14. A source of the P channel transistor 14 is connected to one terminal of a capacitor 16 for holding a voltage in addition to an output terminal 18. The other terminal of the capacitor 16 is connected to ground.
In the charge pump, the same pulse signal is supplied to of the gates of both the transistor 10 and the transistor 16.
In this type of circuit, a voltage VDD is set in the shift capacitor 12 while the transistor 10 is turned on and the transistor 14 is turned off. By shifting the voltage of the shift capacitor 12 by, for example, the voltage VDD using a pulse voltage of VDD while the transistor 10 is turned off and the PMOS 14 is turned on, a voltage of the capacitor 16 is set to the voltage of 2VDD which will be output.
In such a charge pump, current is controlled by measuring an output voltage and adjusting, for example, a frequency of a clock to be supplied according to a measured value of the output voltage.
As the charge pump, one example is disclosed in Japanese Patent Laid-Open Publication No. Hei 7-298607.
However, in the charge pump (a voltage booster circuit) for generating a high voltage necessary for rewriting to a nonvolatile memory or the like, a consumed current and an output current increase almost proportionately with a value of the source voltage VDD as long as an operation clock for the charge pump remains constant regardless of changes in source voltage. Because the output current of the charge pump is proportional to the source voltage, the charge pump is designed to achieve a sufficient amount of the output current for rewriting being secured with a guaranteed minimum source voltage. As a result, regardless of the fact that the amount of current necessary for rewriting to a nonvolatile memory does not change sharply in a region where the source voltage is high, an unnecessarily large amount of current is output when the source voltage is high. At that time, a large amount of source current is thus consumed in vain.